Exobiology in the Solar System & The Search for Life on Mars - ESA

surpris<strong>in</strong>g. To achieve more than Pathf<strong>in</strong>der, an optical spectrograph must have an
extremely high signal-to-noise and even <strong>in</strong> this case it is arguable whe<strong>the</strong>r an
unambigious identification of a m<strong>in</strong>eral could be made. O<strong>the</strong>r techniques (such as
Mössbauer spectroscopy) should be <strong>in</strong>vestigated.
Mössbauer spectroscopy can provide a clear <strong>in</strong>dication of <strong>the</strong> composition of iron
compounds and FE-bear<strong>in</strong>g m<strong>in</strong>erals. It is discussed fur<strong>the</strong>r with<strong>in</strong> <strong>the</strong> Team II report
<strong>in</strong> Section II.5.7.4.
II.6.14.1 Objectives
IR spectroscopy <strong>in</strong> <strong>the</strong> range 0.8-4.0 µm can be used to determ<strong>in</strong>e <strong>the</strong> abundances of
many types of m<strong>in</strong>erals, <strong>in</strong>clud<strong>in</strong>g clays, hydrates and carbonates. Carbonate
materials (such as magnesite, calcite and siderite) can easily be dist<strong>in</strong>guished from
-
one ano<strong>the</strong>r. All have near-IR features <strong>in</strong> <strong>the</strong> 1.7-2.5 µm range attributable to <strong>the</strong> CO3 anion. Sulphates such as gypsum and anhydrite, have absorptions <strong>in</strong> <strong>the</strong> 1.0-1.5 µm
and 4.0-4.7 µm ranges. Ferrous absorptions of pyroxenes are also evident <strong>in</strong> <strong>the</strong> 1.0-
2.0 µm region. <strong>The</strong> ratio of <strong>the</strong> 1 µm to 2 µm absorptions is diagnostic of <strong>the</strong>
composition. Although oliv<strong>in</strong>e has only been found <strong>in</strong> one martian meteorite, <strong>the</strong>
structure of <strong>the</strong> broad absorption feature is also a diagnostic of composition. <strong>The</strong>
absorptions of C-H and C-N bear<strong>in</strong>g organics can be found throughout <strong>the</strong> 1.7-4.0 µm
range. A spectral resolution of >100 (λ/∆λ) would be sufficient. Good spatial
resolution is needed to <strong>in</strong>vestigate variations with<strong>in</strong> a sample and we estimate that a
resolution of 200 µm should be atta<strong>in</strong>able.
A secondary objective <strong>for</strong> IR spectroscopy would be <strong>in</strong>vestigation of <strong>the</strong>
atmosphere. CO 2, H 2O and CO all have characteristic absorption bands <strong>in</strong> <strong>the</strong> IR. <strong>The</strong>
variations of <strong>the</strong>se gases toge<strong>the</strong>r with <strong>the</strong> dust cycle are strongly coupled <strong>in</strong> <strong>the</strong>
martian atmosphere and hence this could provide significant additional <strong>in</strong><strong>for</strong>mation
on <strong>the</strong> time variability of <strong>the</strong> atmosphere. Local sources of <strong>the</strong>se gases may be
present, <strong>in</strong>dicat<strong>in</strong>g hydro<strong>the</strong>rmal activity.
It should be noted that <strong>the</strong> highest resolution near-IR spectroscopic measurements of
Mars come from <strong>the</strong> Hubble Space Telescope, at around 200 km px -1 . <strong>The</strong>se data show
variations <strong>in</strong> ferric alteration m<strong>in</strong>erals and primary ferrous silicates but are limited by
<strong>the</strong>ir coarse spatial resolution. It is not clear whe<strong>the</strong>r we will see low contrast <strong>in</strong> high
spatial resolution data (as was seen <strong>in</strong> <strong>the</strong> optical with <strong>the</strong> Mars Pathf<strong>in</strong>der imager) or
whe<strong>the</strong>r IR spectroscopy at <strong>the</strong> 200 µm level will be extremely reveal<strong>in</strong>g.
II.6.14.2 Requirements
Low-mass IR spectrometer systems have been proposed <strong>for</strong> several missions,
<strong>in</strong>clud<strong>in</strong>g <strong>the</strong> RoLand lander on <strong>the</strong> Rosetta mission, and some systems are now
commercially available. <strong>The</strong> system may require illum<strong>in</strong>ation devices (such as LEDs)
<strong>in</strong> order to compensate <strong>for</strong> absorption l<strong>in</strong>es <strong>in</strong> <strong>the</strong> solar cont<strong>in</strong>uum at <strong>the</strong> surface
caused by dust <strong>in</strong> <strong>the</strong> atmosphere. <strong>The</strong> system should, however, be comparable <strong>in</strong>
mass and power consumption to <strong>the</strong> microscope.
Because transmission IR spectroscopy may not be applicable (requir<strong>in</strong>g
preparation of th<strong>in</strong> sections), <strong>the</strong> less diagnostic reflection mode may have to be
selected. Even <strong>in</strong> this mode, however, identification of m<strong>in</strong>eral and organic
compounds and detection of H 2O and (OH) <strong>in</strong> silicates should be relatively easy. On
<strong>the</strong> o<strong>the</strong>r hand, a Raman microspectrometer should probably provide most of that
<strong>in</strong><strong>for</strong>mation <strong>in</strong> a more unambiguous way.
II.6.15.1 Objectives
<strong>The</strong>rmal-IR spectroscopy <strong>in</strong> <strong>the</strong> 6-9.5 µm region can be used to <strong>in</strong>vestigate specific
absorptions/emissions of carbon-bear<strong>in</strong>g molecules. Strong features are evident at
6.18 µm result<strong>in</strong>g from <strong>the</strong> C-C and C-O bonds, 6.86 µm (-CH2-, -CH3 scissor),
team III: <strong>the</strong> <strong>in</strong>spection of subsurface aliquots and surface rocks/II.6
II.6.13 Mössbauer
Spectroscopy
II.6.14 IR Spectroscopy
II.6.15 <strong>The</strong>rmal-IR
Spectroscopy
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